Raman spectroscopic insights into the transition to the post-post-perovskite phase in NaZnF3

The phase transition sequence of perovskite materials is an important subject in numerous fields ranging from geophysics to material science. The fluoride NaZnF3, adopting the distorted perovskite (Pv) structure (GdFeO3-type), is a model system for studying such transitions. Up to date compression experiments on this compound, limited to pressures of 40 GPa, have identified a perovskite to post-perovskite (pPv, CaIrO3-type structure) transition to occur above 11 GPa followed by yet another transition commencing at 25 GPa and leading to a post-post-perovskite (ppPv) phase whose structure is yet to be identified. Here we present high-pressure Raman spectroscopic study aided by Density Functional Theory (DFT) calculations to explore the phase transition sequence of NaZnF3 at pressures reaching 70 GPa. We confirm that NaZnF3 undergoes a gradual phase transition between the GdFeO3 and CaIrO3 phases starting at 12 GPa. Our results also indicate that above 31 GPa, the CaIrO3 polymorph slowly transforms to the ppPv phase which we unambiguously confirm to adopt the metastable Sb2S3-type structure despite the ground state structure above 23 GPa being La2S3 as predicted from calculations reported in literature. The transition into Sb2S3-type structure type instead of the La2S3 polytype is most probably due to kinetic effects which also hinder the predicted decomposition of NaZnF3.